In a typical rotary clamp cylinder, a cylinder body incorporates a piston therein, and a piston rod connected with the piston projects out from one end of the cylinder body. In the rotary clamp cylinder of this type, a plurality of guide members provided to the cylinder body are respectively engaged with a plurality of turning guide grooves provided in the piston rod, such that the piston rod is turned as well as moved in the axial direction, due to movement of the piston, so as to clamp a work by using a clamper provided to the piston rod. In the past, the guide members have been known as engaging balls (steel balls), as described in JP 3621082 B (Patent Document 1), or known as pins each having a distal end oriented in the direction toward the axis of the piston rod, as described in JP 2000-87909 A (Patent Document 2).